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NixOS specific feature: specialisations

Written by Solène, on 29 August 2022.
Tags: #nixos #nix #tweag

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Credits §

This blog post is a republication of the article I published on my employer's blog under CC BY 4.0. I'm grateful to be allowed to publish NixOS related content there, but also to be able to reuse it here!

License CC by 4.0

Original publication place: Tweag I/O - NixOS Specialisations

After the publication of the original post, the NixOS wiki got updated to contain most of this content, I added some extra bits for the specific use case of "options for the non-specialisation that shouldn't be inherited by specialisations" that wasn't convered in this text.

NixOS wiki: Specialisation

Introduction §

I often wished to be able to define different boot entries for different uses of my computer, be it for separating professional and personal use, testing kernels or using special hardware. NixOS has a unique feature that solves this problem in a clever way — NixOS specialisations.

A NixOS specialisation is a mechanism to describe additional boot entries when building your system, with specific changes applied on top of your non-specialised configuration.

When do you need specialisations §

You may have hardware occasionally connected to your computer, and some of these devices may require incompatible changes to your day-to-day configuration. Specialisations can create a new boot entry you can use when starting your computer with your specific hardware connected. This is common for people with external GPUs (Graphical Processing Unit), and the reason why I first used specialisations.

With NixOS, when I need my external GPU, I connect it to my computer and simply reboot my system. I choose the eGPU specialisation in my boot menu, and it just works. My boot menu looks like the following:

NixOS specialisation shown in Grub

You can also define a specialisation which will boot into a different kernel, giving you a safe opportunity to try a new version while keeping a fallback environment with the regular kernel.

We can push the idea further by using a single computer for professional and personal use. Specialisations can have their own users, services, packages and requirements. This would create a hard separation without using multiple operating systems. However, by default, such a setup would be more practical than secure. While your users would only exist in one specialisation at a time, both users’ data are stored on the same partition, so one user could be exploited by an attacker to reach the other user’s data.

In a follow-up blog post, I will describe a secure setup using multiple encrypted partitions with different passphrases, all managed using specialisations with a single NixOS configuration. This will be quite awesome :)

How to use specialisations §

As an example, we will create two specialisations, one having the user Chani using the desktop environment Plasma, and the other with the user Paul using the desktop environment Gnome. Auto login at boot will be set for both users in their own specialisations. Our user Paul will need an extra system-wide package, for example dune-release. Specialisations can use any argument that would work in the top-level configuration, so we are not limited in terms of what can be changed.

NixOS manual: Configuration options

If you want to try, add the following code to your configuration.nix file.

specialisation = {
  chani.configuration = {
    system.nixos.tags = [ "chani" ];
    services.xserver.desktopManager.plasma5.enable = true;
    users.users.chani = {
      isNormalUser = true;
      uid = 1001;
      extraGroups = [ "networkmanager" "video" ];
    };
    services.xserver.displayManager.autoLogin = {
      enable = true;
      user = "chani";
    };
  };

  paul.configuration = {
    system.nixos.tags = [ "paul" ];
    services.xserver.desktopManager.gnome.enable = true;
    users.users.paul = {
      isNormalUser = true;
      uid = 1002;
      extraGroups = [ "networkmanager" "video" ];
    };
    services.xserver.displayManager.autoLogin = {
      enable = true;
      user = "paul";
    };
    environment.systemPackages = with pkgs; [
      dune-release
    ];
  };
};

After applying the changes, run "nix-rebuild boot" as root. Upon reboot, in the GRUB menu, you will notice a two extra boot entries named “chani” and “paul” just above the last boot entry for your non-specialised system.

Rebuilding the system will also create scripts to switch from a configuration to another, specialisations are no exception.

Run "/nix/var/nix/profiles/system/specialisation/chani/bin/switch-to-configuration switch" to switch to the chani specialisation.

When using the switch scripts, keep in mind that you may not have exactly the same environment as if you rebooted into the specialisation as some changes may be only applied on boot.

Conclusion §

Specialisations are a perfect solution to easily manage multiple boot entries with different configurations. It is the way to go when experimenting with your system, or when you occasionally need specific changes to your regular system.